[PATCH] fcache: add file system frontend cache

[PATCH] fcache: add file system frontend cache

fcache is a frontend cache for a file system that aims to reduce a
dominantly repeatable seeky read workload to sequential access. It does
this by first running the workload in a priming mode, logging the
accesses to the file systme on a cache device. Subsequent runs in
non-priming mode then access the data sequentially from the frontend
cache, instead of punting to the real device.

The primary intended application is to reduce boot time by attaching
to the root file system.

Signed-off-by: Jens Axboe <axboe@kernel.dk>